1. Field of the Invention
This invention pertains to the general field of optical metrology. In particular, the invention relates to a method for materially increasing the speed of focusing in optical profilometry.
2. Description of the Prior Art
Various optical systems and techniques are known to map the height of a sample surface. These include, without limitation, low-coherence interferometry, confocal microscopy, bright-field and dark-field microscopy (image sharpness techniques), and structured light techniques. These methods are all encompassed by what is generally referred to in the art, interchangeably, as optical metrology, optical profilometry, or 3-D microscopy. In the case of low-coherence interferometry (including structured light metrology), the optical signal captured by the system is fringes that yield a process signal referred to as modulation. In the case of confocal microscopy, the optical signal is irradiance that is typically processed as such. In the case of bright-field and dark-field microscopy, the optical signal is irradiance that is normally processed in terms of its standard deviation within neighboring pixels.
In all of these techniques the measurement procedure requires the sample surface to be in focus with the objective of the measuring instrument, that is, at a fixed focal distance from the objective. The in-focus position is normally found by scanning the object through the focal point of the objective while monitoring the light signal detected through the objective lens. As the sample surface approaches the focal point, the signal becomes stronger and the focal position is thus established for the measurement. For example, in the case of interferometric measurements that produce modulated interference fringes during the scan, the focal position is found by determining the point of maximum modulation.
A major drawback of this through-focus scanning approach is the fact that it requires a scan covering the entire span of height variations in the sample surface, which may be in the order of millimeters and therefore require a starting point that is significantly distant from the in-focus position. This need to scan through the entire height span is a time consuming operation that greatly slows down the measurement. In interferometric profilometry this problem is often attenuated by first performing a faster low-magnification scan to find the approximate in-focus position, and then by performing a local through-focus scan at a higher magnification to refine the in-focus placement of the sample for performing the measurement. However, this approach still requires an initial scan through the entire height range of the sample with its attendant time requirements.
Another problem lies in the fact that, because the light signal seen through the objective is strong only when the sample surface is in the vicinity of the focal point, unless the focusing scan happens to start close to focus, it is normally necessary to scan through a significant distance just to establish in which direction the scan needs to move in order to find the in-focus position. This means that the initial focusing scan normally performed in the art is equally likely to move in the wrong as in the right direction. This is particularly crucial when objectives with a short working distance are used because, unless particular attention is paid, the objective may inadvertently crash into the sample while searching for the focus position.
These problems are especially severe when automatic focusing mechanisms and algorithms are used, which is the norm when optical profilometry is used for quality control purposes. Any improvement that accelerates the speed of measurement and decreases the chances of the objective crashing into the sample is a welcome step forward in the art. This invention achieves both of these objectives.